Requirements for input/output ("I/O") pin connections between circuit cards and motherboards often exceed the available circuit card edge length and exceed the maximum available connector pin density. One solution to this problem is the use of a supplemental card portion (i.e., a so-called "mezzanine" card or "daughter" card) that is mounted to the main circuit card in order to provide one or more additional connectors and additional I/O pins. Such mezzanine cards are also used whenever multiple circuit cards are advantageously interconnected for connection to a motherboard, for any reason.
It has been discovered, however, that the use of such mezzanine cards can create an additional challenge because the connectors on the main circuit card and the mezzanine card must be spaced at a carefully controlled distance so that the circuit card and mezzanine card connectors can be mated properly with the mating connectors that are positioned on the motherboard. The spacing between the connectors on the circuit card and the mezzanine card must therefore be held to a relatively tight tolerance in order to ensure proper connection to the motherboard without undo interference. In fact, the required dimensional tolerance for the relative positions of the circuit card connectors, which of course depends on the particular connector system selected, can be as small as about +/-0.002", or even smaller. When these dimensional tolerances are not met, significant structural problems can arise.
It is particularly difficult to maintain such a tight tolerance in view of the fact that the boards used to create the circuit card and the mezzanine card are often formed from a multi-layer substrate. It has been discovered that such substrates can have a typical thickness tolerance of up to about +/-0.007", or even more. The significant variations in the thickness of the circuit card and the mezzanine card present a significant problem for conventional mezzanine connector assemblies.
An example of a conventional mezzanine connector assembly is illustrated in FIG. 1 and is designated by the numeral "10." It has a circuit card 12 and a mezzanine card 14 spaced apart from one another by a conventional standoff 16. Screws 18, 20 engage circuit card 12 and mezzanine card 14 adjacent to the end surfaces of spacer 16. The length of spacer 16 is indicated in FIG. 1 as "L.sub.1."
Circuit card 12 has an upper surface 13a and a lower surface 13b, and mezzanine card 14 has an upper surface 15a and lower surface 15b. Upper and lower ends of spacer 16 abutt against lower surface 15b of mezzanine card 14 and against upper surface 13a of circuit card 12, respectively. The length L.sub.1 of spacer 16 sets the distance between upper surface 13a of circuit card 12 and lower surface 15b of mezzanine card 14.
The tolerance of length L.sub.1 of spacer 16 can be carefully controlled by standard machining processes. However, the thickness T.sub.12 of circuit card 12 and the thickness T.sub.14 of mezzanine card 14 can vary from card-to-card or even at various positions on a single card. This is especially true when a laminated, multi-layered card is used. For example, multi-layered copper clad board is formed from a multi-layered substrate with copper coatings on the facing surfaces of the substrate. The substrate itself (such as so-called "FR 4" board, for example) is often formed from layers of glass fiber mesh impregnated by epoxy. Such fiber reinforced boards have been known to vary within a broad thickness tolerance up to or even exceeding about +/-0.007".
In view of the tolerances contributed by the thickness of the circuit card 12 and/or the mezzanine card 14, the distance D.sub.1 (between upper surface 13a of circuit card 12 and upper surface 15a of mezzanine card 14) is difficult to control within a tight tolerance. If spaced connectors are mounted on upper surface 13a of circuit card 12 and on upper surface 15a of mezzanine card 14, then the spacing of those connectors will vary depending upon the actual thickness T.sub.14 of mezzanine card 14. In other words, distance D.sub.1 is the combined distance of length L.sub.1 of spacer 16 and thickness T.sub.14 of mezzanine card 14, and the dimensional tolerances of the spacer 16 and mezzanine card 14 are cumulative. Accordingly, variations in the thickness T.sub.14 of mezzanine card 14 contribute to variations of the spacing between connectors mounted on surfaces 13a and 15a.
The same problem arises with respect to the distance D.sub.2 (between lower surface 15b of mezzanine card 14 and lower surface 13b of circuit card 12) because of the built-up of tolerances resulting from length L.sub.1 of spacer 16 and the thickness T.sub.12 of circuit card 12. This variation would therefore contribute to variations of the spacing between connectors mounted on surfaces 13b and 15b. A further accumulation of tolerances would result if connectors were mounted on surfaces 13b and 15a because two substrate thicknesses (T.sub.12 and T.sub.14) would contribute to the overall distance between those surfaces.